A strapping machine is known, for example, from DE 196 02 579 A1. Besides the familiar parts that are present in such machines, such as machine frame, work bench, and band guide frame for leading the strapping band around the object being strapped as a loose loop, it has a band drive unit placed underneath the work bench. This comprises a combined shooting and retrieval unit for shooting the strapping band into the band guide frame and retrieving the strapping band from the band guide frame until the strapping band bears against the object being strapped. Moreover, a back-tensioning mechanism is often present for tightening the strapping band about the object, being dependent on the stack height.
The shooting and retrieval unit has at least one pair of rollers, between the gap of which the strapping band is led through by one of the rollers in the shooting and retrieval direction. The back-tensioning unit has another pair of rollers, between the gap of which the strapping band is likewise fed and can be clamped around the item being strapped in the machine by at least one of the rollers in the retrieval direction.
Finally, guide channel sections are provided to guide the strapping band through the band drive mechanism, which lead the strapping band brought up from a supply roll or a temporary storage device to the back-tensioning mechanism, between the latter and the shooting and retrieval unit, and from the latter in the direction of the band frame on the work bench. The guide channel sections are formed by cheeks which guide the strapping band on either of its flat sides, being configured as webs or side surfaces of larger prismatic bodies.
A typical problem in the operation of such a strapping machine sometimes occurs due to a faulty handling of the strapping band. Thus, for example, the leading end of the band cannot reach the welding head due to obstacles in the band guide frame and therefore the detector situated here, for example, in the form of an end switch, is not activated to detect the leading end of the band at the end of its shooting trajectory in the band guide frame. The control system of the strapping machine recognizes this and it then arranges for a retrieval of the strapping band, usually until it emerges backward from the pair of drive rollers of the band drive mechanism opposite the shooting direction.
This fault situation is normally monitored by a sensing mechanism, which detects the condition when the strapping band emerges from the shooting and retrieval unit opposite the shooting direction.
To remedy the fault in the strapping machines known thus far it was necessary to open the band drive mechanism and manually thread the strapping band into the shooting and retrieval unit, so that the shooting process can start over again. This leads to a substantial down time for the strapping machine, which is basically undesirable and in an extreme case can lead to a shutdown of the entire production line when the strapping machine is integrated as part of a continuous production process for print products, for example.
In EP 1 489 005 A2 a corresponding method is indicated for actuating a band drive mechanism that can eliminate the described fault situation without manual intervention in substantially less time. According to this, the back-tensioning mechanism can be reversed in its drive direction so that when a fault is detected—that is, a band loss in the shooting and retrieval unit—the strapping band is transported back almost automatically to the shooting and retrieval unit.
A drawback with this control method is the fact that the fault situation and the driving of the band must be detected by means of a suitable sensor, e.g., an incremental encoder on the pressing roller of the shooting and retrieval unit. This signal is then further processed in the control program and incorporated into the control sequence. This requires additional expense for control technology and apparatus.
This drawback is even more glaring for the strapping machine of DE 603 18 160 T2, which is different in its design makeup of shooting, retrieval and back-tensioning unit. Here, the band length that was shot out and retrieved in the fault situation is detected and the band is pulled back with two different band speeds to the starting position before being threaded once again.